Plasma processes are widely used for applying and etching thin films, particularly in the manufacture of semiconductors where micron and sub-micron thick layers of conductor, semi-conductor and insulator material are deposited, stacked and etched, in repeated cycles. In such manufacture, the properties of the plasma must be carefully controlled to maintain the quality, uniformity and consistency of the processed films. Precise control of the plasmas requires exact knowledge of the state of the plasma being controlled.
Optical methods are widely used to monitor the properties of plasmas and states and courses of plasma processes, particularly in the semiconductor manufacture industry. Observable radiation emitted from a plasma contains much information regarding the nature of the plasma present in a vacuum chamber that can be used by an operator or a programmed controller in the control of the plasma process. For the information to be used, the radiation emitted from the plasma must be accurately measured.
Optical emission spectroscopy is a measurement technique used in plasma processes to measure a full spectrum of radiation emitted from a plasma over a given range of wavelengths, particularly those that include visible and near visible light. This measurement technique involves some optical element such as a transparent lens or window through which spectroscopy sensors can observe the plasma and read emissions from the plasma for measurement and analysis. A light transmissive window is typically used to separate the plasma process chamber from the optical detection system and to allow the spectroscopy equipment to be situated in an ordinary atmosphere environment away from the vacuum environment within the chamber containing the plasma.
In the course of plasma vacuum processes that involve deposition and etching, vapors of various coating materials, reactants, etch byproducts, and other materials tend to fill the vacuum space within the chamber. Eventual deposition of coatings on the view windows through which optical detectors observe the plasma is common. Such deposits affect the window transparency and the radiation being monitored, and can affect the accuracy of the optical measurements if the change in window transparency is not considered.
When the windows through which optical measurements of plasma processes become clouded with film, they can and ultimately must be cleaned or replaced for the plasma process to continue. Such window maintenance involves processing system down-time, which is expensive.
In United States Patent Application 2005/0173375, a related application identified above, an apparatus and method for use of an optical system with a plasma processing system is disclosed. In this published application, the disclosed system and method are provided in conjunction with a plasma processing system, and the system is constructed and arranged to detect a plasma process condition through the window as well as the transmission condition of the window. The method includes detecting an optical emission from the plasma processing region and monitoring contamination of the window through which the plasma is being observed by the optical system.
In addition to the monitoring the contamination of the window through which a plasma is being observed, the simplicity and reliability of the instrumentation, the length of the time between window servicings, and the accuracy and usefulness of the monitored information, affect the quality and efficiency of the plasma process. Accordingly, constant improvement is needed in optical monitoring and control systems for plasma processes, particularly in semiconductor manufacture.